Pentaquark discovery confounds sceptics

By Hazel Muir, New ScientistJuly 2, 2003

A brand new sub-atomic particle called the pentaquark has made its debut at labs in Japan and the US. Unlike ordinary protons and neutrons in atomic nuclei, which contain three quarks, the pentaquark has five.

The result has delighted Russian physicists who predicted the mass of the particle in 1997, but met a lot of scepticism from their peers.

"It was not an easy decision to publish our paper six years ago, but eventually we went ahead despite resistance in the community," says Maxim Polyakov, now at the Ruhr University in Bochum, Germany. "It is a great pleasure that our theory seems to be correct."

The pentaquark may have been common in the Universe just after the Big Bang, 14 billion years ago. And further studies of it could help patch up some holes in the theory of the strong force that glues quarks together in particles like protons and neutrons.

"The discovery is not just getting another animal in a zoo," says Polyakov. "It will seriously influence our understanding of what the ordinary proton and neutron are made of and 'how they work'."

Up and down

Particles that contain quarks fall into two main categories. "Baryons", such as stable protons and neutrons in atomic nuclei, contain three quarks. "Mesons" contain two, a quark and an anti-quark, but they are never stable and vanish in a split second.

Theory does not forbid the existence of a short-lived five-quark particle, and scientists have looked for them in the debris of particle-smasher experiments for decades. Having turned up nothing, they were beginning to think they had missed some rule of nature that bans pentaquarks from forming.

But they got a new lead in 1997, thanks to work by Polyakov, Dmitri Diakonov and Victor Petrov at the Petersburg Nuclear Physics Institute in Russia. They predicted that one particular pentaquark — containing two "up" quarks, two "down" quarks and an "anti-strange" quark — should be about 1.5 times as heavy as a proton.

Now scientists say they have spotted a particle with the right mass and all the hallmarks of a pentaquark. A team led by Takashi Nakano of Osaka University and another led by Ken Hicks at the Jefferson lab in Virginia made a high-energy gamma ray interact with a neutron to create a meson and a pentaquark. The pentaquark survived for only about 10-20 second before decaying into a meson and a neutron.

The Japanese results appear this week in Physical Review Letters. Experiments at a Moscow lab have also found evidence for this pentaquark. "The absence of these multiquark particles has bothered physicists for the last forty years," Polyakov told New Scientist. "Now it is over."

But for the moment, physicists say they know very little about the new particle. "The discovery of the pentaquark is really too new," says Hicks. "We haven't had time to think about the implications."